It is a mutation because the Homo Sapien evolved from a different specie wich means there were genetic mutations to make us be able to survive the current habitat.
Look at Inuits ( Eskimoes if you prefer) The fat that babies are born with changes when they turn to adults but the Inuits fat type never changes when they turn adults because the fat is more usefull for living in cold climates, A genetic mutation.

It may have once been a mutation, but what I'm saying is that once it's in the gene pool and starts behaving normally, it isn't a mutation anymore. Saying that it is a mutation because it once was is wrong. The fact means that every time a child has blue eyes, something happened out of the norm, or the fact is wrong. Idk one way or the other if it is true about blue eyes, but that's what the terminology means.